For increasing data transmission performance and reliability, the so-called multiple input and multiple-output technology (MIMO) may be used in wireless radio frequency telecommunications for transmitting information between a base station and a user equipment. The MIMO technology relates to the use of multiple send and receive antennas for a wireless communication at for example a base station or a user equipment. The MIMO technology forms the basis for coding methods which do not only use the temporal dimension but also the spatial dimension for transmitting information and, therefore, enables a space and time coding. Thus, the quality and data rate of the wireless communication may be increased.
When a large number of user equipments is arranged within a cell served by a base station having a plurality of antennas and transmitting information according to the above-described MIMO technology, such an arrangement is called a massive MIMO system. In such a massive MIMO system, the configuration of the individual antenna transceivers of the base station may vary depending on the location of each of the user equipments and transmission conditions in an environment of the base station and the user equipment.
A massive MIMO system may be used in connection with a time division duplex (TDD) system in which a transmission of an information stream between the base station and a user equipment is split up into time slots embedded in a frame structure. Different time slots for uplink (UL) data communications and downlink (DL) data communications may be provided for communicating information from the user equipment to the base station (uplink) and for communicating information from the base station to the user equipment (downlink). In such a massive MIMO system, there is a need for an additional time slot which may be called a “header” for transmitting a pilot signal or a training signal including a training sequence from the user equipment to the base station. Based on the received pilot signal, which is also called “footprint” of the user equipment, the base station may configure the transceivers of its antenna array according to spatial and environmental conditions. Thus, high antenna gain for the payload to be transmitted in the following time slots can be achieved. The payload may be transmitted in a number of uplink and downlink time slots. However, when the user equipment is moving the channel quality may degrade due to a change of the spatial arrangement of the base station and the user equipment.
Typically, massive MIMO systems are expected in buildings such as offices, shopping malls and so on. In this environment a large number of user equipments is expected. The mobility of the user equipments or a changing spatial arrangement of the base station and the user equipments may demand that channel training sequences are sent frequently in order to keep up with the aging or erosion of the antenna configurations of the MIMO system. The faster a user equipment is moving, the more frequently a training sequence needs to be transmitted from each user equipment to the base station. Typically, the base stations in such a massive MIMO system are configured in a way to allow a maximum speed or spatial arrangement change of the user equipments. However, this may have an impact on the system throughput as the frequently sent training sequences may occupy a valuable and significant part of the data communication channel.
For a typical user connected to a MIMO system it is likely that the pilot signal transmission intensity is proportional to the velocity of the user. In addition, the higher the number of users that are moving, the less there are pilots available, and hence less numbers of users can be supported by the system. A problem that may arise in such an implementation is when the user is not moving but is using the mobile device for example for browsing and/or in a talk mode. The user is stationary, but the corresponding mobile device is not as a mobile device in the hands of a user is always randomly moved within a limited space nearby the user. The mobile device's position and orientation may change a little back and forth as the user is tapping on the display of the mobile device or when user is showing something to a friend. An impact of this constantly present, random movement is that, to maintain good connection, the pilot information needs to be sent very often, so that the system may be overloaded with pilot information more than needed.
Therefore, it is the object of the invention to provide an improved method of operating abuse station in a MIMO system as well as a corresponding base station, which allow to avoid or at least reduce an overload of the system due to excessive pilot or training signal transmissions caused by movements of a user equipment in the system, especially by small movements within a limited space near the respective user.